Pakistani authorities on Saturday were searching for an insurgent figure believed to have aided five Northern Virginia men who allegedly tried to join al-Qaeda, saying the case could help unravel a growing network of terrorist recruiters who scour the Internet for radicalized young men.

Investigators have identified the man, known as Saifullah, as a recruiter for the Pakistani Taliban and said he contacted one of the American men on YouTube, exchanged coded e-mails with the group, invited them to Pakistan and guided them once they arrived.

But the men, all Muslims from the Alexandria area, failed to reach the remote tribal zone that is al-Qaeda's home because the terrorist network's commanders thought they were sent by the CIA to infiltrate al-Qaeda -- and Saifullah could not convince them otherwise, a Pakistani intelligence official said Saturday.

"They were regarded as a sting operation. That's why they were rejected," said the official, who spoke on the condition of anonymity because of the sensitivity of the investigation. The five men disappeared just after Thanksgiving and were arrested near Lahore on Tuesday. They have not been charged with any crime.

The developments point to the dangers posed by an extensive and sophisticated network of online terrorist recruiters, but also its limitations. Investigators and terrorism experts say recruitment worldwide has become far more Web-based, with recruiters playing a critical role in identifying potential radicals and determining whether they can be trusted.

Yet Saifullah's endorsement, secured through months of online contact with the five men, apparently did not carry much weight with Osama bin Laden's organization: It wanted someone who knew them better.

As a result, the five men wound up marooned in the eastern city of Sargodha, far from the terrorist haven in the forbidding mountains of northwest Pakistan that they were apparently trying to reach. Pakistani officials said the men were undeterred and kept trying to acquire the endorsements to gain access to al-Qaeda training camps -- with the ultimate goal of fighting U.S. troops in Afghanistan -- when they were arrested.

The men, ages 18 to 24, traveled overseas without telling their families, triggering an international manhunt after concerned relatives contacted the FBI. The five -- Ramy Zamzam, 22; Ahmad A. Minni, 20; Umar Chaudhry, 24; Waqar Khan, 22; and Aman Hassan Yemer, 18 -- were transferred Saturday from Sargodha to Lahore, where they were questioned by the FBI.

The U.S. ambassador to Pakistan, Anne W. Patterson, met with Pakistani President Asif Ali Zardari on Saturday to discuss the men and the timing of what officials said will be their eventual handover to the United States. But Pakistani officials said they want more time to question the men in an effort to learn more about Saifullah and other radicals they may know.

U.S. law enforcement officials are considering criminal charges against the men, but they said that no charges are imminent and that a decision on whether to file them could take weeks. The young men's friends and spiritual advisers have said they never saw any sign of radical activity or beliefs. The men's family members in Northern Virginia have declined to comment.

If the emerging case, as outlined by Pakistan officials, shows the difficulties online recruiters can encounter, it was also clear that the growth of online recruiting poses unique challenges for U.S. criminal investigators.

Federal officials said they were aware of the threat and concerned about its potential to radicalize Americans who might meet recruiters online, both Muslims and non-Muslims.

"Online recruiting has exponentially increased, with Facebook, YouTube and the increasing sophistication of people online," a high-ranking Department of Homeland Security official said Saturday on the condition of anonymity because the investigation is ongoing.

But criminal investigators said the explosion of online communication made it extraordinarily difficult to monitor, and they indicated that their tracking abilities were limited by constitutional and privacy considerations. "Other countries may have different capabilities, and those are capabilities we don't have," said one federal law enforcement official, who was not authorized to speak publicly about the issue.

Ironically, terrorism experts said one reason for the growth of online recruiting is the success of efforts by the United States and other nations to penetrate Islamist terrorist networks and Muslim communities since the Sept. 11, 2001, attacks.

"Increasingly, recruiters are taking less prominent roles in mosques and community centers because places like that are under scrutiny. So what these guys are doing is turning to the Internet," said Evan Kohlmann, senior analyst with the U.S.-based NEFA Foundation, a private group that monitors extremist Web sites.

Since Sept. 11, U.S. intelligence has made it a top priority to place human assets inside al-Qaeda. The organization's recruiters act as gatekeepers, keeping out those who are not serious about their commitment to holy war as well as those who could be spies.

Would-be American recruits are treated with special scrutiny by al-Qaeda, analysts said. But they are also considered enormously appealing to the group because of their potential to access U.S. targets and because of their propaganda value.

But experts said terrorist organizations have become much more cautious in recent years about who they allow in as U.S. intelligence agencies grow increasingly knowledgeable about the groups' recruiting methods.

Terrorist group operatives, and even freelance recruiters, troll jihadi social-networking sites, attempting to establish relationships with young men who seem ideologically committed, and physically able, to commit violence in the name of radical Islam.

In one case, a recruiter named Younes Tsouli is thought to have used such sites to identify dozens of aspiring insurgents for the war in Iraq -- all without leaving his London basement.

Experts said the case of the Northern Virginia men is especially troublesome because it apparently involved recruiting on YouTube, a Web site with mass appeal that is extremely difficult to monitor.

Pakistani officials have said that Saifullah first contacted one of the men, Minni, on YouTube in August after Minni repeatedly praised YouTube videos showing attacks on U.S. forces.

A Pakistani police official involved in the investigation said Saifullah and the men exchanged coded e-mails for months thereafter. After their arrival in Pakistan, he advised them to wear the local dress and instructed them to take buses to a city near the edge of the tribal areas, from where they could be transported to North Waziristan, home base of al-Qaeda. They were arrested before they could make the journey.

The men have told investigators that Saifullah was the only one who welcomed them in Pakistan and that they were rejected by at least two other extremist groups.

Pakistani investigators say they believe that Saifullah spent time in the United States, because of his familiarity with American slang and geography. Officials said he was already wanted for his alleged role in an attack this year on the Sri Lankan cricket team as it visited Lahore for a tournament.

In most cases, experts said, potential recruits are the ones who reach out to radical Web sites and chat rooms in the hopes of finding someone to introduce them to a militant group.

"A recruiter does not radicalize a person from scratch," said Manuel R. Torres Soriano, a terrorism expert in Spain, where the Internet played a key role in influencing some of the perpetrators of the 2004 Madrid train bombings. "They deal with people who are already ready to die."

Witte reported from Kabul, Markon from Washington and Hussain from Sargodha. Special correspondent Haq Nawaz Khan in Peshawar, Pakistan; correspondents Craig Whitlock in Berlin and Sudarsan Raghavan in Madrid; and staff writer Joby Warrick in Washington contributed to this report.